In a typical solid fueled rocket motor, illustrated in FIGS. 1 and 2, a case 10 of metal or reinforced polymeric material is provided to which a nozzle 12 is attached. Within case 10 is a propellant grain 14 having a burning surface 16 and an outer or nonburning surface 18. (When surface 18 is referred to as a "nonburning" surface, this means that burning does not propagate from surface 18. Instead, the propellant grain burn propagates from surface 16. The entire propellant grain, including the portions defining surface 18, ultimately will burn.) Outer surface 18 is bonded to the inner surface 20 of case 10 to provide structural integrity and to make outer surface 18 a nonburning surface. The necessary bonding between surfaces 18 and 20 is facilitated by a liner 22 typically having a polymeric base compatible with the binder of grain 14, adhering to nonburning surface 18 and insulation 24. Insulation 24 is directly bonded to inner surface 20. In some structures or parts of structures, liner 22 is bonded directly between nonburning surface 18 and inner surface 20.
The motor is constructed by providing case 10, placing insulation 24 where needed to protect the casing, applying a liner 22 to all interior surfaces of the insulation and casing which are to receive the propellant, precuring liner 22, and then casting propellant grain 14 within the casing. The propellant grain is then cured and becomes bonded to the liner. Optionally, a sheet or layer of inhibitor material can be bonded to any exposed surface of the propellant intended to be a nonburning surface. (The composition of an inhibitor is similar to that of a liner.)
Propellants sometimes contain various nitrate esters, and some propellants consist mainly of nitrate esters. Nitrate esters serve as energetic binders, explosives, and plasticizers, and burn to produce a large volume of relatively low molecular weight gas. However, nitrate esters can decompose when the propellant grain is stored, particularly when exposed to heat. The products of decomposition accelerate further decomposition, so once started the decomposition process is self-catalyzing. The decomposition products of nitrate esters can also convert the carbamate linkages in urethane based propellant binders to gaseous products which form cracks or voids in the grain. To prevent nitrate ester decomposition and increase the storage life of the cured propellant grain, propellants usually contain a homogeneously mixed nitrate ester stabilizer. Heretofore, all of the stabilizer has been mixed into the propellant grain uniformly so it would be available where the compounds to be stabilized are found.
However, the present inventors have discovered that nitrate esters in a propellant grain do not decompose uniformly. The nonburning surface 18 of the grain, which is confined by the casing, decomposes most quickly. Decomposition proceeds more quickly near the casing because the decomposition products cannot escape easily, so they remain in place and catalyze further decomposition. Another possible contributing factor is interaction of the binder with liner ingredients. If enough stabilizer is homogeneously disposed in the grain to prevent decomposition adjacent to the casing, the rest of the grain will contain excess stabilizer. The stabilizer displaces more energetic components, and unnecessarily increases the weight of the propellant grain.
Furthermore, when the stabilizer is mixed uniformly at relatively low levels in the propellant grain, approximately 10 to 30 percent of the stabilizer reacts with the isocyanate cure agent of the isocyanate binder, and so is unavailable to prevent nitrate ester decomposition. This increases the amount of stabilizer which must be added to the propellant to leave an effective level of stabilizer after the propellant is cured, when little or no free isocyanate remains in the propellant.
As a result of nitrate ester decomposition, rocket motors age and must be replaced periodically to ensure reliable performance. Also, all nitrate esters used in a propellant grain must be relatively pure to avoid premature decomposition.